Looking at the actions of the men and women of ancient Greek mythology, it is sometimes easier to come up with the people involved in the betrayal than who betrayed whom.
Apate is the name of the goddess of deceit in Greek mythology, a child of Night (Nyx), and the sister of Eris (Strife), Oizus (Pain), and Nemesis (Retribution). Together these pained and paining ladies represent a multitude of the negative features of human existence, all of whom are met in ancient stories of betrayal.01of 07
Jason and Medea
Christian Daniel Rauch Public domain or Public domain, via Wikimedia Commons
Jason and Medea both violated each other's expectations. Jason had lived with Medea as her husband, even producing children, but then put her aside, saying they were never married, and that he was going to marry the local king's daughter.
In retaliation, Medea slew their children and then flew away in one of the classic instances of a deus ex machina in Euripides' Medea.
There was little doubt in ancient times that Medea's betrayal was greater than Jason's.02of 07
Atreus and Thyestes
Which brother was worse? The one who engaged in the family sport of cooking children or the one who first committed adultery with his brother's wife and then raised a son for the purpose of killing his uncle? Atreus and Thyestes were sons of Pelops who himself had once been served up as a feast to the gods. He lost a shoulder in the event because Demeter ate it, but he was restored by the gods. Such was not the fate of the children of Thyestes whom Atreus cooked. Agamemnon was a son of Atreus.03of 07
Agamemnon and Clytemnestra
Like Jason and Medea, Agamemnon and Clytemnestra violated each others' expectations. In the Oresteia trilogy the jury couldn't decide whose crimes were more heinous, so Athena cast the deciding vote. She determined that Clytemnestra's murderer was justified, even though Orestes was Clytemnestra's son. Agamemnon's betrayals were the sacrifice of their daughter Iphigenia to the gods and bringing back a prophetic concubine from Troy.
Clytemnestra (or her live-in lover) murdered Agamemnon.04of 07
Ariadne and King Minos
When the wife of King Minos of Crete, Pasiphae, gave birth to a half-man, half-bull, Minos put the creature in a labyrinth built by Daedalus. Minos fed it the youth of Athens who were paid to Minos as annual tribute. One such sacrificial youth was Theseus who caught the eye of Minos' daughter, Ariadne. She gave the hero a string and a sword. With these, he was able to kill the Minotaur and get out of the labyrinth. Theseus later abandoned Ariadne.05of 07
Aeneas and Dido (Technically, not Greek, but Roman)
Since Aeneas felt guilty about leaving Dido and tried to do so secretly, this case of jilting a lover counts as a betrayal. When Aeneas stopped at Carthage on his wanderings, Dido took him and his followers in. She offered them hospitality and in particular, offered herself to Aeneas. She considered theirs a commitment like a betrothal, if not a marriage, and was inconsolable when she learned he was leaving. She cursed the Romans and killed herself.06of 07
Paris, Helen, and Menelaus
This was a betrayal of hospitality. When Paris visited Menelaus, he became enamored of the woman Aphrodite had promised him, Menelaus' wife, Helen. Whether Helen was in love with him, as well, is unknown. Paris left Menelaus' palace with Helen in tow. To regain Menelaus' stolen wife, his brother Agamemnon led the Greek troops to war against Troy.07of 07
Odysseus and Polyphemus
Crafty Odysseus used trickery to get away from Polyphemus. He gave Polyphemus a goatskin of wine and then poked out his eye when the cyclops fell asleep. When Polyphemus' brothers heard him roaring with pain, they asked who was hurting him. He answered, "nobody," since that was the name Odysseus had given him. The cyclops brothers went away, mildly puzzled, and so Odysseus and his surviving followers, clinging to the under-bellies of Polyphemus' sheep, were able to escape.